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Science: Astronomers mystified by galaxy evolution

By JOHN FAIRALL in Melbourne

THE MAGELLANIC clouds, two nearby galaxies, are undergoing rapid star
formation, according to an astronomer in Australia. This flies in the face
of theories of how galaxies evolve. Star formation should occur rapidly
at first, when a galaxy is young, then level off as gas, the raw material
of stars, runs short.

Gary Da Costa, of the Anglo Australian Observatory, presented the results
of his observations at a recent meeting of the International Astronomical
Union in Sydney. The meeting was convened principally to discuss the Milky
Way’s satellite galaxies: the Large Magellanic Cloud (LMC) and the Small
Magellanic Cloud (SMC). The clouds are small and irregular and among the
few galaxies close enough for us to distinguish individual stars.

In the accepted theory of galactic evolution, gravity causes massive
clouds of dust and gas to clump together to form galaxies. At first, stars
are born at a relatively steady rate but, as the galaxy ages, star formation
begins to slow.

Observations confirm that this has indeed happened to the Milky Way.
But, according to Da Costa, the Magellanic Clouds have not evolved in such
a straightforward way at all.

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Da Costa tracks the rate of star formation in a galaxy over time by,
first, establishing the age of its star clusters. These are aggregations
of tens of thousands of stars, all born at more or less the same time. Although
it is difficult to assign an age to an individual star, it is a lot easier
to carry out a statistical analysis with many thousands.

Next, for each cluster, Da Costa measures the amount of heavy elements
– ones heavier than hydrogen and helium, and dubbed ‘metals’ by astronomers.
This is usually expressed as the ‘metal ratio’, the ratio of the abundance
of these elements compared to hydrogen. It depends only on the metal ratio
in the cloud from which the primordial cluster condensed. So, by looking
at clusters of different ages in a galaxy, it is possible to determine how
the metal ratio of the gas has changed over the life of the galaxy.

The metal ratio is important because it indicates the rate of star formation
in a galaxy. Astronomers assume that the primordial galactic gas was mostly
hydrogen but that, over time, heavy elements have been created inside stars
and returned to the void, so enriching the gas with metals. So, the change
in the metal ratio over time is a pretty good indication of the rate of
star formation.

In the Milky Way, metal production was greatest early in its history,
but then levelled off, pretty much as predicted by theory. But in the two
Magellanic Clouds, it did not happen like this at all (see Figure).

In the LMC, the oldest clusters show the metal level rising steeply.
But then, the supply of clusters stops. Astronomers know of only one cluster
that is between 12 billion years old and 4 billion years old. At 4 billion
years the process resumes vigorously, and cluster formation continues to
this day.

The SMC shows yet another pattern. Da Costa says that the metal ratio
changed little until about 3 billion years ago. At that time, there appears
to have been a substantial increase in the rate of metal enrichment.

As with the LMC, astronomers are at a loss to explain this pattern of
behaviour. But there may be some clues. Matiku Fujimoto, from Negoya University
in Japan, presented the results of a computer model that looks at the dynamics
of the Milky Way and the two Magellanic Clouds. The model shows that they
collided recently, and will collide again soon.

Fujimoto suggests that tidal disruption between the three galaxies might
well have altered fundamentally the evolution of the two small systems.
At present, a huge tail of gas, known as the Magellanic Stream, is being
ripped from the LMC by the Milky Way. There seems to be a similar tail of
gas, called the Magellanic Bridge, connecting the SMC and LMC.

Bill Shuter from the University of British Columbia has developed an
alternative model. He attributes these features to a collision with the
Andromeda galaxy, M31. The Andromeda galaxy is another member of the Local
Group of galaxies. Shuter’s model says that the collision would have occurred
about 6 billion years ago.

Other evidence presented at the conference showed that the SMC has a
quite bizarre structure. It seems to be warped like a boomerang, with one
arm perpendicular to the line of sight, and the other receding directly
away from us. It is not rotating, and astronomers find it difficult to see
and understand how it all keeps together.

However, it may be that other explanations will emerge for the differences
between the three galaxies. Some astronomers suggest that we still do not
understand the mechanics of star formation. For instance, astronomers do
not understand what switches it on or off.